For our tests, we used volunteers who say they face taxi discrimination themselves when accompanied by service dogs or using a wheelchair.

The entire WUSA9 investigation is caught on camera.

Of 42 cabs tested, using passengers with wheelchairs or guide dogs, 20 cabs - or 48% - either drove right past the passenger with a disability in favor of another fare, took them to the wrong location without warning, or charged an illegal extra fee.

That's nearly twice as many underserved passengers as another undercover WUSA9 investigation which showed 25% of Washington cabbies passing black passengers in favor of white passengers 100 feet down the street.

In one case, while we were leaning inside a taxi asking him why he'd refused service to a blind woman with a service dog, he peeled off - forcing our undercover reporter out - driving away with the door still open.

One woman was dropped off hundreds of feet away from the Constitution Avenue entrance to the National Gallery of Art she had requested without warning.

"You're right in front of the entrance," Melanie Brunson recounted the cabbies words. "I specifically said Constitution, and he said yes."

Upon asking tourists to tell her where she was, she learned she was at the right building but on the wrong street and a block away from the address she requested, where our undercover operation was staged.

Another cab added a $1.50 extra charge to the blind woman's tab while only disclosing the full fare due.

Brunson, who was one of three undercover passengers during our test, is executive director for the American Council of the Blind.

A different cab dropped off American Council of the Blind advocate Eric Bridges at the wrong building on the wrong street.

"It happens probably a few times a month," Bridges said about drop-offs at wrong locations. "DC and the cab system here is a bit like the Wild West -it's a challenge that we're going to continue to face."

One cab company defended refusing service to our undercover wheelchair passenger, saying the wheelchair wouldn't fit in the trunk their full-size sedans.

Despite that claim, much smaller compact cabs did pick up the passenger and fold his wheelchair in the trunk.

DC Taxi Commission officials say cabs are required to either pick the passenger up themselves, or notify dispatch to send a specially equipped van.

When confronted, nearly every cabby that passed the highly visible disabled passengers claimed he did not see the disabled passengers.

On camera, one cab is shown stopped by traffic right in front of Bridges while he was hailing a cab.

The cab picked up speed as soon as traffic cleared and offered a ride a hundred feet down the street to the WUSA9 decoy that didn't have a service dog or wheel chair.

"I thought he was going to pick him up," the driver said when confronted, claiming he thought the cab in front of him would pick them man up. "I was waiting for him," he claimed although video shows him a hundred feet away from the stranded passenger with a service dog.

"This is a critical problem and we all need to address it," Don Kahl with the Equal Rights Center said. "It's illegal in a number of ways."

Kahl group did a similar test in 2010 showing 60% of passengers using guide dogs received discriminatory service.

When it released the 2010 results, the Equal Rights Center called on enforcement from D.C. government, mandatory training for taxi companies, and compliance monitoring.

The chairman of the DC Taxi Commission says the agency is reliant on passengers to complain and that more than 75% of complaints that go to arbitration result in fines against the driver.